I have a '96 cavalier that makes a humming noise in the gas tank within 5 seconds of starting. I have tried two fuel pumps but the noise is still there!!!

When you turn the Key to the ON Position, prior to the Start Position: The Fuel Pump engages - making a humming noise. This is normal. The Fuel Pump is priming the Fuel Pressure in the System.
After 5-seconds of keeping it in the ON Position and not advancing it to Start, the Fuel Pump will shut down.

Let me know if this helped, or if you have additional information or questions. Feel Free to contact me at FixYa.com!

This is normal.when you turn the key on the fuel pump will go on for a few seconds and shuts off until you turn the key to the start position.All G.M.s do this.Some pumps are louder than others.Hope this helps.Good luck.

Your fuel pump is supposed to hum, when you turn the key it primes the system and it will hum. When the car is running the fuel pump is running and it will hum. If the noise is excessive then try a higher quality fuel pump, it may be less noisy. You will hear the fuel pump on a lot of vehicles tho when you are sitting at a stop light with your radio off and everything quiet in the car.